Method for manufacturing an eyeglass lens

ABSTRACT

A method for manufacturing a spectacle lens, in which at first a semi-finished uncut spectacle lens (hereinafter referred to as a blank) is produced, i.e. a spectacle lens having merely one finished optical surface (hereinafter described as base surface); subsequently a prescription-optimized surface is computed according to the data of a spectacle lens prescription; and then the prescription-optimized surface is finished according to the computed data. The invention is characterized by the following method steps: after the production of the semi-finished spectacle lens the base surface is measured; the prescription-optimized surface is computed and finished taking into account not only the individual data of the spectacle prescription, but also the actual shape of the base surface.

TECHNICAL FIELD

The invention relates to a method of manufacturing a spectacle lens, asset out in the preamble of patent claim 1.

STATE OF THE ART

Apart from very few exceptions—e.g. directly casting a spectacle lensfinished on both sides—at present the manufacture of a spectacle lensfor correcting eye defects is performed as follows:

At first, a semi-finished and uncut spectacle lens which, for example,is round and unedged is fabricated. A spectacle lens of this kind,having merely one finished optical surface which hereunder is designatedas a base surface, and one as yet unfinished second surface, isdesignated also as a blank (see ISO Standard 10322). This blank whichrepresents a—commercially available—semi-finished product is fabricatedeither “as stock” or at short notice because of a definite order, or ispurchased by a manufacturer of a prescription-optimized surface fromanother manufacturer. This kind of purchase is not uncommon in so-calleddecentralized fabrication in which use is made of prescription-lensworkshops.

As soon as a definite order, based on a spectacle lens prescription, isreceived by a manufacturer of prescription-optimized surfaces, aprescription-optimized surface is computed, or a “fitting” prescriptionsurface is selected from available previously computed prescriptionsurfaces, in accordance with the data of the particular spectacleprescription. This prescription surface is then fabricated according tothe computed or selected previously computed data.

Because the fabrication machinery of so-called “prescription-lensfabricating works” is designed for processing concave and therewitheye-side surfaces as an rule the front surface is chosen to be the basesurface. A prescription-optimized surface or selected prescriptionsurface is therefore (as a rule) a concave eye-side surface.

Until recently, in practice the surface which is more complicated toproduce was predominantly chosen to be the base surface, i.e. theprogressive surface for a progressive spectacle lens, or the toroidal oratoroidal surface for a single-vision lens with astigmatic power.

For example, in the case of progressive spectacle lenses, from several10 to more than 100 different progressive front surfaces, differing fromeach other in surface power at the distance reference point (base curve)and addition power (increase of power from the distance portion to thenear portion), were computed (in advance, i.e. irrespective of anyindividual wearing position) and fabricated as blanks (on stock). Toconform a blank to a given spectacle prescription, a concave eye-sidesurface was then produced on the semi-finished spectacle lens providedwith the progressive front surface. For this, the choice of the eye-sidesurface was made so that the spherical power stipulated by thespectacles prescription was achieved by the values of spherical surfacepower of the front surface and the eye-side surface (prescriptionsurface). In the case of an additional astigmatic power, instead of aspherical or aspherical eye-side surface a toroidal or atoroidal surfacewas fabricated, the cylinder power and cylinder axis of whichcorresponded to the respective prescription values.

Furthermore, it is known to combine a base surface which is designed asa progressive surface and which is also the front surface with anaspherical or atoroidal eye-side surface which has been computedspecially for a specific wearing situation, i.e. for the individualconditions of a particular spectacles wearer. Concerning this, attentionis drawn to DE 42 10 008 A1 or DE 195 11 613 A1, or to the spectaclelenses manufactured by Optische Werke G. Rodenstock, Munich, DE, whichare designated “Multigressive (II)” and have been in prior public use.

However, in the patent literature it has already been proposed for sometime that the surface which is more complicated to produce, for examplea progressive surface, be selected as the prescription-optimized surfaceand designed, for example, in such manner that it additionally also hasan astigmatic power according to the spectacles prescription concerned.Concerning this, attention is drawn to U.S. Pat. No. 2,878,721 or DE 4337 369 A1. A proposal similar to that of the two publications mentionedabove is also contained in DE 197 01 312 A1 and also other patentapplications. According to these applications, the front surface servingas base surface is a rotationally symmetrical and, in particular, aspherical surface.

A spectacle lens based on DE 43 37 369 A1 has been manufactured andmarketed under the trade name “Impression” or “ILT” by the firm ofOptische Werke G Rodenstock, Munich, DE, since May 2000: with thisspectacle lens the prescription-optimized surface is the eye-sidesurface which is designed to be progressive and has been computedaccording to prescription values as well as individual data of thespectacles wearer concerned (interpupillary distance, vertex distance,pantoscopic angle and further individual data) and also, as the case maybe, a chosen spectacles frame. A spherical, aspherical, or atoroidalfront surface is used as a base surface, the atorus (as a rule) servingonly for an aesthetic matching of the front surface to the shape of thelens rims of a chosen spectacle frame and not for correction of anyastigmatism of an eye.

The wording of the preamble of patent claim 1 sets out from thisspectacle lens, or the method applied in manufacturing this spectaclelens.

Furthermore, reference is expressly made to the above-mentionedpublications and spectacle lenses in prior public use for an explanationof all details not described here more closely.

Now the fabrication of spectacle lenses is beset with defects offabrication—as is any fabrication. In the manufacture of rotationallysymmetrical (single power) spectacle lens surfaces, or toroidal orprogressive surfaces, the manufacturers of blanks as a rule do notproduce a particular finished face of a blank with the highest possibleprecision that would be usual, for example, with lenses for precisionoptical systems such as photographic objective lenses, but for reasonsof cost (only) with markedly lower precision. The deviations of surfacepower value and surface astigmatism occurring with commerciallyavailable semi-finished products are—as has been found according to theinvention—frequently even greater than the values permissible accordingto ISO Standard 10322:

According to ISO 10322, for single power spectacle lenses havingspherical surfaces and a vertex surface power between 2 and 10 dpt, themaximum spherical deviation from the vertex power, and also the maximumsurface astigmatism at every point of the surface, is of an amount up to±0.06 dpt. With progressive surfaces, the spherical and astigmaticdeviations from stipulated values may be even greater. For blanks, inparticular of a highly refracting plastics material, surface astigmatismvalues of up to 0.25 dpt resulting from distortion effects and the likecan be found on the periphery, the occurrence of distortion phenomenaand the resulting surface astigmatism being subject to statisticalfluctuations.

In JP 10-175 149 A it has therefore been proposed to select, forfinished base surfaces which are the surfaces more complicated toproduce, such as progressive surfaces, suitable eye-side surfacesfollowing a merely “cursory” measurement.

A similar manner of proceeding has been described for contact lenses inU.S. Pat. No. 4,980,993.

However, none of the two aforementioned publications describe spectaclelenses for which a prescription-optimized surface is individuallycomputed with the respective data of a spectacles wearer, and for whichthe computation is modified on the basis of the measurements. Both ofthese publications therefore describe other methods than the onespecified by the preamble of patent claim 1.

DESCRIPTION OF THE INVENTION

In accordance with the invention it has been realized that even thetypically occurring and apparently small deviations of spectacle lenssurfaces from the stipulated values must in no way be neglected, andthat they may have a not insignificant effect, for example on theso-called visus. This, of course, is even more the case when the actualdeviations are greater than the deviations admissible according to ISO10322, and attain values of 0.25 dpt and even more.

“Visus” designates the reciprocal of the angular acuity of vision; theangular acuity of vision being the smallest angle which can be resolvedby an eye. The visus thus represents a measure of the power of visionachieved by a spectacles wearer on the basis of the system“spectacle-lens/eye”. Concerning the relationship between the value ofthe visus and the properties of a spectacles lens, attention is drawn to“Forschungsbericht Visus” (“Research Report Visus”) of the Institute forMedical Optics of the University of Munich, to which furthermorereference is expressly made concerning an explanation of the term“visus” used here and all other details.

The invention is based on the object of developing further a method formanufacturing a spectacle lens, in which first a semi-finished and uncutspectacle lens is fabricated, subsequently the prescription optimizedsurface is computed in accordance with the data of a spectacles lensprescription, and then the prescription-optimized surface is finishedaccording to the computed data, in such manner that the visus achievedwith the finished spectacle lens for the system spectacle-lens/eye isnot impaired by fabrication-caused properties of the base surface.

An achievement of this object is set out in patent claim 1. Furtherdevelopments of the invention are the subject matter of claim 2 andthose following.

In accordance with the invention it has been realized that even“apparently” small defects of fabrication, such as occur in prior artwith spherical or simple toroidal front surfaces of semi-finishedspectacle lenses, have an unexpectedly large effect on the visus,especially when the prescription-optimized surface is an individuallycomputed progressive surface. Particularly with cast blanks, thesefabrication defects can nullify the advantages which individualcomputation has for the visus.

According to the invention a method of the generic kind is thereforefurther developed in such manner that following the production of asemi-finished spectacle lens the base surface is measured. Theprescription-optimized surface is computed by taking into account notonly the data of the prescription, but also by taking into account theactual shape of the base surface, i.e. particularly the deviations ofthe actual values of the base surface from the theoretically desiredvalues—which means that it is not selected from given surfaces—and it isthen finished. With this manner of proceeding there is obtained, on thebasis of the prescription-optimized surface which has been changed—inaccordance with the physical deviations—, an actual shape of thecontours of equal visus, as obtained theoretically by computing thesurface using the theoretical values of the base surface, even when thebase surface actually present on the blank seriously deviates from the(stipulated) theoretical base surface,

The method of the invention is of advantage particularly when theprescription-optimized surface is computed by taking into account notonly the basic optical data of a spectacles wearer (spherical power,astigmatism, cylinder axis of the astigmatism), but taking into accountalso the individual data (interpupillary distance, vertex distance,pantoscopic angle etc.), and possibly also, as the case may be, theshape of the rims of a chosen spectacles frame. In this case, a newcomputation of the prescription-optimized surface is always necessaryfor the fabrication of each spectacle lens, so that an additional outlayfor the manufacture of the spectacle lens of the invention is limited toa measurement of the base surface.

This manner of proceeding is cheaper than a fabrication of a basesurface with appropriately greater accuracy than is at present customaryin spectacle optics.

In principle the following combinations of base surface andprescription-optimized surface are possible:

Progressive Power Lenses Prescription-Optimized Base Surface Surface 1.Progressive Rotationally Symmetrical Surface Asphere or Atorus 2. SphereProgressive-Atoroidal 3. Torus Progressive-Atoroidal 4. ProgressiveProgressive-Atoroidal Surface

Within the scope of the present application a progressive surface isunderstood to be an aspheric surface which provides a significantcontribution to the power increase of a spectacle lens. An atoroidalsurface is understood to be an aspherical surface having one, two or noaxes of symmetry, which provides no significant contribution to thepower increase of the whole spectacle lens, and frequently, but notnecessarily, provides a contribution to the correction of an astigmatismof an eye. A progressive-atoroidal surface is understood to be a surfacewhich with astigmatic prescriptions provides a substantial contributionboth to the power increase and to the astigmatic power of the spectaclelens. In the case of a spherical prescription, the progressive-atoroidalsurface in principle corresponds to a progressive surface, however inany case all possible parameters such as, for example, vertex distance,interpupillary distance etc. are taken into account in the computationof the progressive surface.

Single Power Lenses Prescription-Optimized Base Surface Surface 1.Rotationally Sym- Rotationally Symmetrical metrical Asphere or Asphereor Atorus Atorus 2. Sphere Rotationally Symmetrical Asphere or Atorus 3.Torus Rotationally Symmetrical Asphere or Atorus

The method of the invention is however specially preferred for spectaclelenses in which the prescription-optimized surface is a progressivesurface, i.e. a surface, the power of which in the wearing positionchanges between at least two regions. In accordance with the inventionit has been found that for surfaces of this kind, small deviations ofthe actually present base surface from an ideal base surface stipulatedin the computation of the progressive surface already lead to aconsiderable reduction of the visus. This applies particularly when theprogressive surface also provides any astigmatic power which may berequired in accordance with an individual spectacles prescription; inthis case as a rule the progressive surface is computed “on demand” whena definite order is received, so that—as has already been explained—anyadditional outlay is restricted to a measurement of the (frequently, butin no way absolutely necessary) spherical base surface.

However, it is also possible for the prescription-optimized surface tobe an atoroidal surface having two, one or no axes or planes ofsymmetry. In many cases however, even then the base surface will have anat least approximately spherical or rotationally symmetrical asphericshape.

In accordance with the invention the base surface which is either instock, or fabricated in advance, or may even be purchased, is providedand measured. The measurement can be performed either point by point atthe reference points, or over the entire surface, the last-mentionedmethod being preferred. Especially with blanks which have been producedby casting, peripheral local deviations from the desired value data mayoccur because of distortions which are not detected by a measurement ofpeak values, or by a measurement at the reference points.

The measurement and the analysis of any given physically manufacturedsurface is followed by a fitting of a theoretical surface to themeasurement values. An individual computation and an optimization of aprovided prescription-optimized surface with respect to the data of aspectacles wearer is performed by taking into account the results ofmeasurement on the base surface. In the case of a point by pointmeasurement it is preferred for the prescription-optimized surface to becomputed by means of support positions which coincide with themeasurement points.

Now, the spectacle lens provided by the invention always has the bestimaging properties regardless of the quality of the base surface,because the fabrication defects of the base surface are taken intoaccount in the computation of the prescription-optimized surface andthus compensated.

Because, as a rule, the base surface is intended to be a semi-finishedproduct which is as inexpensive as possible, the demands made on thequality of the base surface can therefore be appreciably be lowered. Amarked price-advantage is therefore obtained together with a betterquality of the entire spectacle lens. However, it is not necessary forthe base surface to be a semi-finished product; it must only be measuredin every case before the prescription-optimized surface is computed.

Necessary for the method according to the invention is a rapid method ofmeasurement and a rapid method of computation and optimization of theprescription-optimized surface. On modern computers an optimization ofthe prescription-optimized surface can be performed in less than oneminute.

Methods of measurement that can be employed with the method of theinvention are practically all known methods which are suitable for themeasurement of surfaces of spectacle lenses. Various usable methods aredescribed for example in the review articles “Der Einsatz modernerMessmethoden zur Entwicklung und Fertigung von Brillengläsern” (TheEmployment of Modern Methods of Measurement for the Development andFinishing of Spectacle Lenses”) by Dr. Wolfgang Grimm, published in DOZ,December 1984, Page 41 ff., or in “Messtechnische Gesichtspunkte zuoptisch anspruchsvollen Brillengläsern” (Aspects of MeasurementTechnique for Optically Exacting Spectacle Lenses), Dr. Günther Guilino,published in DOZ December 1988, Page 8 ff. Furthermore, an excellentlysuitable method for the computation method of the invention is themeasurement of the base surface with a reflection measurement method.Interferometric methods are also well suited, because they detectprecisely the deviations of the actual surface from the theoreticallydesired surface.

Apart from a general improvement of the imaging quality and a reductionof manufacturing costs, yet further advantages of the method of theinvention result:

With small prescribed cylindrical powers, small errors in the referencepoints of the base surface become very strongly noticeable. As will yetbe described in the following, with the cross cylinder method it ispossible to obtain an estimate for the resulting cylindrical power fromthe cylindrical power of the base surface and the desired cylindricalpower of the prescription-optimized surface. With this it can be seenthat small astigmatic errors already lead to large rotations of thecylinder axis.

By taking into account this error which is known in ophthalmic optics ina computation of the prescription-optimized surface, this rotation ofthe axis can now be avoided.

SHORT DESCRIPTION OF THE DRAWINGS

In the following the invention will be described by way of example,without limitation of the general inventive concept, with reference tothe drawings to which, furthermore, attention is expressly drawnconcerning the disclosure of all details of the invention not describedmore explicitly in the text. Shown by:

FIG. 1 are the isometric lines of the visus for a theoretical basesurface and a corresponding progressive-atoroidal surface;

FIG. 2 are the isometric lines when the base surface is a fabricatedspherical surface having deviations approaching a tolerance limit, andthe prescription-optimized surface according to FIG. 1 is used;

FIG. 3 are the isometric lines when the base surface according to FIG. 2and a prescription-optimized surface computed for this base surface areused.

DESCRIPTION OF AN EXAMPLE OF EMBODIMENT

In the following, first the various cross-cylinder methods will bedescribed to explain the effects of small errors on surface astigmatism:

Cross Cylinder Method—Additioncyl _(x) =cyl ₁·cos(2·A ₁)+cyl ₂·cos(2·A ₂)cyl _(y) =cyl ₁·sin(2·A ₁)+cyl ₂·sin(2·A ₂)cyl _(res)=√(cyl _(x) ² +cyl _(y) ²)A _(res) =a tan(cyl _(y) /cyl _(x))wherein

cyl₁, A₁ cylinder 1: magnitude and cylinder axis cyl₂, A₂ cylinder 2:magnitude and cylinder axis cyl_(res), A_(res) resulting cylinder:magnitude and cylinder axis

It is therefore possible to stipulate the deviations at the referencepoints, for example for spherical, cylindrical, prismatic powers, in thecomputation of the prescription optimized surface.

Furthermore, it is possible to take into account an error at theposition of an engraving. The semi-finished products are often providedwith an engraving, based on which the finished spectacle lenses arestamp-marked and then fitted into a spectacles mount by anophthalmologist according to the stamp-marks. A shift of theseengravings thus directly affects the wearing comfort of a spectacleswearer. With the method of the invention it is now possible tocompensate this error.

The quality of a spectacle lens is characterized by the spherical andastigmatic deviation. For this, a difference from a stipulatedastigmatism A₀(y) is best computed with the cross-cylinder method whichtakes into account both the magnitude and the cylinder axis.

Cross-cylinder Method—Subtractioncyl _(x) =cyl _(act)·cos(2·A _(act))−cyl _(des)·cos(2·A _(des))cyl _(y) =cyl _(act)·sin(2·A _(act))−cyl _(des)·sin(2·A _(des))cyl _(res)=√(cyl _(x) ² +cyl _(y) ²)A _(res) =a tan(cyl _(y) /cyl _(x))wherein

cyl_(act), A_(act) actual cylinder (spectacle lens): magnitude andcylinder axis cyl_(des), A_(des) desired cylinder (prescription):magnitude and cylinder axis cyl_(res), A_(res) resulting cylinder(astigmatic defect): magnitude and cylinder axis

The cross-cylinder method—subtraction will be explained in detail withthe aid of Example 1.

EXAMPLE 1

A prescription has a spherical power of 0.5 dpt at the distancereference point, a cylindrical power of 2.5 dpt, and a cylinder axis of0 degrees, and also an addition power of 2.0 dpt. The computed spectaclelens has a cylindrical power of 2.5 dpt at one point on the principalline, and a cylinder axis of 2 degrees at one point on the principalline. An astigmatic error of 0.174 results from this.

In the following the invention will be described in detail withreference to FIGS. 1 to 3:

FIG. 1 shows the case that a progressive-atoroidal surface is optimizedfor a theoretical base surface which in this case is a sphere withoutdefects. In the example shown, the base surface which is the frontsurface has a power of 5.12 dpt at the vertex.

At the distance reference point which is illustrated by a circle at y=8mm in FIG. 1, the spherical power of the spectacle lens is 0.5 dpt. Theaddition power, i.e. the difference of the optical powers at thedistance reference point and the near reference point (circle at y≈−14mm), is 2 dpt. The principal line winds towards the nose at leastbetween the distance reference point and the near reference point

In FIG. 1 the isometric lines of the visus—without accommodation by thespectacles wearer—are shown for an unfinished round spectacle lenshaving a radius of 30 mm, and a typical edged spectacle lens, a visus of1 being the “normal case” for an eye having correct-sight, or for a“corrected” eye. It can be seen clearly that the distance portion iscomparatively large, and that the visus line 0.9 extends into theperiphery of the distance portion at a comparatively low level.

FIG. 2 shows the case in which the front surface is not a theoretical,defect-free spherical surface, but an actually fabricated sphericalsurface having defects. The deviations of the spherical surfaceaccording to FIG. 2, which is used, are slightly larger than specifiedby ISO 10322 and correspond to the typical deviations of sphericalsurfaces, such as those offered cheaply by outside providers for use infinishing prescription lenses. The progressive atoroidal surface used inthe example according to FIG. 1, which has been computed or optimizedfor the theoretical base surface, is used as the prescription-optimizedsurface.

It can be clearly seen that the isometric line of the visus 0.9 nolonger extends into the distance portion at a level as low as in FIG. 1,but “penetrates” into the distance portion at a higher level.

FIG. 3 shows an example of embodiment of the invention in which the basesurface is the fabricated spherical surface according to FIG. 2, havingdeviations approaching the tolerance limit. Again, theprescription-optimized surface is a progressive atoroidal surface whichhowever has been specially optimized for the base surface actually usedand measured before the optimization. The isometric lines of the visusextend exactly as in the “ideal case” according to FIG. 1; inparticular, the isometric line of the visus 0.9 extends exactly at justas low a level as with the purely theoretical example according to FIG.1.

With this it has surprisingly been found that the tolerances of afabricated progressive-atoroidal surface which has been computed for aphysical base surface beset with defects, do not have as large an effecton the visus as the tolerances of the base surface. Thus in practice aspectacle lens which has been substantially improved in comparison withprior art is obtained by matching the progressive-atoroidal surface or,in general, the prescription-optimized surface to a physical andmeasured base surface. This manner of proceeding is less costly thaninitially fabricating the base surface as exactly as possible,especially when a whole-surface and rapid measurement of the basesurface is performed. Furthermore, a combination of a measured basesurface and a matching prescription-optimized surface is more tolerantof errors than a combination of a base surface which has been fabricatedas exactly as possible and a theoretically prescription-optimizedsurface.

The invention has been described above with the aid of an example ofembodiment without limitation of the general inventive concept which maybe derived from the present application and the claims.

1. Method for manufacturing a spectacle lens, in which initially asemi-finished uncut spectacle lens or blank is produced, including aspectacle lens having merely one finished optical surface or basesurface; subsequently a prescription-optimized surface is computedaccording to data of a spectacle lens prescription; and then theprescription-optimized surface is finished according to the computeddata; wherein: after production of the semi-finished spectacle lens thebase surface is measured; the prescription-optimized surface is computedand finished taking into account individual data of the spectaclesprescription, wherein the computed prescription-optimized surface takesinto account deviations of actual values of sagittal heights of the basesurface from theoretical desired values.
 2. Method according to claim 1,characterized in that the deviations of actual values of sagittalheights of the base surface from theoretical desired values taken intoaccount are deviations present on the entire surface.
 3. Methodaccording to claim 2, characterized in that the prescription-optimizedsurface is computed taking into account no only basic optical data ofthe spectacles prescription (spherical power, astigmatism, cylinder axisof the astigmatism), but also taking into account individual data(interpupillary distance, vertex distance, pantoscopic angle etc.) of aspectacles wearer and possibly also, as the case may be, a shape of lensrims of a chosen frame.
 4. Method according to claim 3, characterized inthat the prescription-optimized surface is a progressive surface, i.e. asurface, a power of which in the wearing position changes between atleast two regions.
 5. Method according to claim 4, characterized in thatthe progressive surface also provides any astigmatic power which isnecessary according to the individual lens prescription.
 6. Methodaccording to claim 5, characterized in that the prescription-optimizedsurface is an atoroidal surface.
 7. Method according to claim 6,characterized in that the base surface is a rotationally symmetricalsurface or an atoroidal surface, a shape of which has been chosen foraesthetic reasons for matching the shape of lens rims, and an astigmaticpower of which, as a rule, does not serve to compensate any astigmatismof an eye.
 8. Method according to claim 6, characterized in that boththe base surface and the prescription-optimized surface are progressivesurfaces.
 9. Method according to claim 1, characterized in that theprescription-optimized surface is computed taking into account not onlybasic optical data of the spectacles prescription (spherical power,astigmatism, cylinder axis of the astigmatism), but also taking intoaccount individual data (interpupillary distance, vertex distance,pantoscopic angle etc.) of a spectacles wearer and possibly also, as thecase may be, a shape of lens rims of a chosen frame.
 10. Methodaccording to claim 9, characterized in that the prescription-optimizedsurface is a progressive surface, i.e. a surface, a power of which inthe wearing position changes between at least two regions.
 11. Methodaccording to claim 10, characterized in that the progressive surfacealso provides any astigmatic power which is necessary according to theindividual lens prescription.
 12. Method according to claim 9,characterized in that the prescription-optimized surface is an atoroidalsurface.
 13. Method according to claim 1, characterized in that the basesurface is a rotationally symmetrical surface or an atoroidal surface, ashape of which has been chosen for aesthetic reasons for matching theshape of lens rims, and an astigmatic power of which, as a rule, doesnot serve to compensate any astigmatism of an eye.
 14. Method accordingto claim 13, characterized in that the base surface has an at leastapproximately spherical shape.
 15. Method according to claim 1,characterized in that both the base surface and theprescription-optimized surface are progressive surfaces.
 16. Methodaccording to claim 15, characterized in that the measurement of the basesurface is made point by point.
 17. Method according to claim 15,characterized in that the measurement of the base surface is performedwith an interferometric or reflective method.
 18. Method according toclaim 15, characterized in that the measurement of the base surface ismade point by point.
 19. Method according to claim 18, characterized inthat a theoretical surface which is used for computing theprescription-optimized surface is derived from measured points. 20.Method according to claim 18, characterized in that theprescription-optimized surface is measured by means of support pointswhich coincide with the measured points.